The loss of sensory hair cells from the human ear is a leading cause of hearing and balance disorders. Although the potential for regeneration in the human ear is very limited, the ears of nonmammalian vertebrates can quickly regenerate after injury. A detailed understanding of the biological basis of this repair process should suggest methods for promoting similar forms of regeneration in humans. The overall goals of this study are to identify the biological signals that regulate hair cell phenotype and orientation during regeneration. One project will focus on the differentiation of vestibular hair cells. It has been known for over 50 years that the vestibular organs of higher vertebrates contain two distinct classes of sensory hair cell (which are known as type I and type II), yet we know nothing about the developmental origin of these two cell classes. Using newly-described markers for these cells, we will carry-out a quantitative characterization of the recovery of type I and type II hair cells during regeneration. A second project will examine how hair cell stereocilia reacquire their proper orientation during regeneration. We have found that regenerated hair cells in organ cultures of the avian ear are normally-oriented and our preliminary data suggest that molecules of the planar cell polarity (PCP) signaling pathway are critically involved in this process. A series of experiments will use in vitro methods to directly test the role of PCP signaling in regeneration. Finally, we will carry-out a detailed study of changes in the expression of core PCP molecules in the mammalian cochlea after ototoxic injury. Such knowledge will be crucial to all current efforts aimed at the induction of biological repair in the mammalian inner ear.